In such a spray drying apparatus, a liquid product is turned into a powder product by contacting an atomized liquid feed with drying air or gas. The drying takes place in a drying chamber, in which the drying air is brought into contact with the atomized feed, and the atomization takes place by one or more nozzles or other forms of atomizer, such as a rotary atomizer. The drying air enters the drying chamber via an air disperser positioned at or near the top of the drying chamber. Further drying means may be provided in such a drying system, for instance a fluidized bed. One and the same spray drying apparatus may be utilized either for drying the same, or a number of different liquid feeds.
The parts of the spray drying apparatus needs to be cleaned at regular intervals, for instance following a predefined cleaning schedule. The regular cleaning is necessary in order to meet the requirements set by governmental regulations and/or manufacture specifications. Such cleaning is carried out at suitable intervals to avoid product degradation, contamination and build-up of deposits in the components. The cleaning normally takes place by means of so-called Cleaning-In-Place (CIP) systems, in order to keep the time needed for cleaning as short as possible with disassembly of as few components as possible. The cleaning fluid may be water or possibly an alternating use of water and suitable detergents or cleaning agents. For some purposes, CIP may be used in combination with a further sterilization process. After CIP, drying out of the systems takes place.
The interior of components such as the drying chamber and other units forming part of a spray drying plant such as cyclones, bag filters, fluid bed chambers, process chambers, containers, tanks, ducts or any similar vessel are cleaned by cleaning nozzles distributing the cleaning fluid into the chamber or vessel itself. In spray drying apparatus utilizing nozzles for atomization of the liquid, the nozzles are removed from the drying chamber for cleaning and placed in CIP stands on top of the drying chamber in order to clean the nozzles, nozzle lances and the upstream feed tubes. Cleaning of the entire spray drying apparatus is normally carried out at standstill, but parts of the apparatus are CIP-cleaned also during operation.
The atomizer nozzles mainly comprise two categories, viz. pressure nozzles and two-fluid nozzles. Many other types of nozzles also exist, including combinations of the above mentioned types. In pressure nozzles, the liquid feed is supplied at high pressure, whereas two-fluid nozzle atomization is achieved pneumatically by high-velocity compressed air/gas impacting the liquid feed. Two-fluid nozzles are suitable for products where flow properties cause sharp increases in viscosity with shear. Such products are normally not suitable for atomization in pressure nozzles, but are successfully handled in two-fluid nozzles, where atomization is achieved by application of very low shear stresses. The kinetic energy for atomization in two-fluid nozzles is supplied by compressed air, and hence, the application of two-fluid nozzles is limited due to the costs of producing the compressed air.
Now dealing with pressure nozzles, during cleaning, for instance as a step in a cleaning schedule, the nozzles, possibly attached to respective nozzle lances, are removed from the drying chamber, typically in groups of for instance 4 to 12 nozzles, and placed in the CIP stands on top of the drying chamber. Before removing a group of nozzles, i.e. a subset of the total number of nozzles in operation, the supply of liquid feed hereto is stopped, for instance by switching supply valves for the feed, and the nozzles, nozzle lances and upstream feed supply tubing are flushed with purge air or gas to empty these. Following the cleaning procedure, the nozzles are reinserted into the drying chamber, and the supply of feed is initiated again, typically all controlled by a control system. This routine is then performed for another subset of nozzles. Such a change-over process of a group of nozzles typically takes up to 30 minutes, depending on the spray dryer size, type and the feed involved. Cleaning of a group of nozzles may e.g. typically be determined by and connected to an upstream cleaning of a related evaporator and/or other feed-processing equipment.
Although such change-over and cleaning procedures function very satisfactory, it has turned out that over time, formation and build-up of deposits on parts of the spray drying apparatus, in particular on the side and top walls inside the drying chamber, are unavoidable. As such deposits are detrimental from a number of points of views, in particular as regards cleaning, product quality, capacity, and even overheating.
Thus, several attempts have been made to reduce the formation of deposits, one example being shown in Applicant's international publication WO 93/23129 A1 (the commercial product being traded under the name JET SWEEP™), and to survey operation to detect deposits, for instance as shown in Applicant's international publication WO 2011/063808 A1 in which monitoring of the spray dryer is carried out by the means of infrared cameras during operation.
However, while the above publications provide several advantages, the issue of avoiding or reducing deposits remains crucial.
Also, pressure nozzles with an integrated valve is known, but for the above mentioned purpose have a row of disadvantages making these a non-option for many purposes in food or pharmaceutical spray drying processes. Such disadvantages are e.g. reliability as to function and tightness, CIP-possibilities, and the danger of emitting pieces from wear-parts into the product.